1. Field of the Invention
The present invention relates to an optical connector for interconnecting optical fibers, and more particularly, to an optical connector for connecting a single-mode fiber to a holey fiber at an optical fiber laying site.
2. Description of Related Art
FIG. 5 is a schematic illustration showing a transversal cross sectional view of a typical holey fiber. As shown in FIG. 5, a holey fiber 51 with a fiber diameter of 125 μm comprises a cladding 52 made of quartz, a core 53 in which germanium (Ge) is added to quartz so that the core 53 has a refractive index slightly higher than that of the cladding 52, and hollows 54 formed around the core 53. Since the holey fiber 51 has a characteristic that is a small increase in transmission loss caused by a bend, the holey fiber 51 has attracted much attention in that it can be wired easily at outdoor sites such as ordinary houses, condominiums, and offices.
Usually, mechanical splices and optical connectors are widely used to interconnect optical fibers at a laying site. In general, it is effective to use a mechanical splice for a permanent connection and to use an optical connector when optical fibers are connected and disconnected frequently. In either case, the optical fibers are physically interconnected by applying a thrust force in the axial direction at ends of both fibers. The optical fiber is generally fragile, so when using an optical connector, the optical fiber is inserted into a ferrule to protect it, enabling the optical fiber end to be physically brought into contact (e.g., see JP-A Hei 08(1996)-114724, U.S. Pat. No. 5,631,985).
FIG. 6 is a schematic illustration showing a longitudinal cross sectional view of an example of a conventional optical connector. As shown in FIG. 6, the main body of the optical connector 61 comprises a ferrule 62, a V-groove board 63, a holding board 64, and a housing 65; an optical fiber 11a is included in the main body. It is proposed that the optical connector 61 connects the optical fiber 11a to another optical fiber 11b by applying a refractive index matching material or adhesive in a liquid or grease state between the optical fiber ends to be interconnected, wherein the refractive index of the refractive index matching material or adhesive has the same or approximately the same refractive index as the cores of the optical fibers 11a and 11b (e.g., see JP-A-2000-241660, JP-A Hei 11(1999)-72641, JP-A Hei 11(1999)-101919, and JP-A Hei 08(1996)-122562).
In other optical connectors, it is known that a solid refractive index matching material, such as a film, is used instead of the above refractive index matching material in a liquid or grease state (e.g., see JP Patent No. 2676705, JP-A-2001-324641, and JP-A Shou 55(1980)-153912).
At an actual optical fiber laying site, a procedure for interconnecting fibers in the optical connector is as follows. FIG. 7 is a schematic illustration showing the conventional optical connector fixed to a jig for interconnecting fibers. FIG. 8 is schematic illustrations showing transversal cross sectional views of the conventional optical connector in a procedure for interconnecting fibers, (a) before inserting wedges; and (b) after inserting wedges into clearances of the optical connector. As shown in FIGS. 7, 8(a) and 8(b), the optical connector 61 is fixed to a jig 71; wedges 72 are inserted into clearances between the holding board 64 and the V-groove board 63; a cut optical fiber is inserted into the V-groove 66; and the inserted optical fiber is connected to the optical fiber pre-included in the optical connector 61 through a refractive index matching material r6. In this procedure, a refractive index matching material or adhesive is applied to the end surfaces of the optical fibers, and the optical fibers are butt jointed by being matched. This interconnection method thereby keeps air out of the connection ends and eliminates Fresnel reflection that would otherwise be caused by air, reducing the connection loss.
However, in the case that the refractive index matching material r6 is used to connect the holey fiber 51, which has hollows, to the optical fiber 11a pre-included in the optical connector 61, when the refractive index matching material r6 may be a silicone or paraffinic one in a liquid or grease state in a conventional method, the refractive index matching material r6 then enters the hollows 54 of the holey fiber 51. The refractive index at the hollows 54 is largely changed by the refractive index matching material r6 entered in the hollows 54, thereby significantly increasing the transmission loss.
There is another problem when the refractive index matching material r6 enters the hollows 54. It is that the amount of refractive index matching material r6 between the fiber ends at the connected part decreases by an according amount, and thus voids and bubbles are likely to be generated between the ends, thereby increasing a connection loss between the fibers.
A method in which a film as a refractive index matching material in a solid state is used instead of one in a liquid or grease state to connect a holey fiber 51 to an optical fiber is advantageous in that the connection loss is small. In this method, however, compression or tensile stress is loaded on the film when, e.g., the ambient temperature changes. The refractive index of the film is changed due to a deformation, i.e., an optical return loss is changed, and therefore stable optical characteristics cannot be obtained.
In another method, the hollows 54 in the end of the holey fiber 51 are sealed before the holey fiber 51 is connected. This method is not suitable to a laying site at which a simple and easy connection is required, because a special device is needed and much time is taken for the treatment, requiring an additional cost.
Furthermore, a fusion splicing method is also available as the connection method. This method is also problematic because a fusing machine is required and the method cannot be applied to connectors used for a simple connection made at a laying site in a general manner.